This is an octopus. It has possesses both short- and long-term memory, can recognize individuals, and practices observational learning. It can solve mazes and basic puzzles, use tools, mimic other animals, and even has a sense of play, which is only observed in higher vertebrates such as birds and mammals. In England, it’s even achieved the status of “honorary vertebrate” under animal testing laws. However, because it is not a true vertebrate, you are no relation to it.
In fact, you are more closely related to this creature:
It’s a tunicate. Specifically, this tunicate is a sea squirt. It has no sense of play, memory, or observational learning. It is not smarter than a 5th grader. It doesn’t own the intellectual capacity to play Candyland. In fact, it doesn’t even have a brain. (Anymore. It doesn’t have a brain anymore, a curiosity we’ll get to in a minute.) But it does, for a brief moment in its life, possess a notochord, which puts it in the phylum Chordata, which makes it the simplest life form to possess something like a spine. Fish, amphibians, reptiles, birds and mammals all owe a debt to this grand-daddy/grand-mama (it’s hermaphroditic) of all vertebrates. Today we’ll pay homage to the humble sea squirt and the bizarre family of early chordates that predate us, still rocking hard 540 million years after they began the movement.
Tunicates are sac-like filter feeders shaped much like one-chambered hearts, with a single chamber serviced by one inhalant “in” siphon and one exhalant “out” siphon. Anatomically, the most important thing that separates the tunicate from other filter feeders such as anemones and mussels is its planktonic stage, in which the free-swimming, tadpole-like larva possesses such familiar organs as an ocellus (a primitive eye), a cerebral ganglion (a primitive brain), and the aforementioned notochord (a primitive spinal cord). The superficial similarity to an actual tadpole is brief, however. Within about 24 hours of developing, the planktonic larva has found a flat, rocky surface to which it anchors itself and begins its metamorphosis into its sessile, tube-like adult form. (Childhood is a fleeting pleasure in the tunicate’s life.) Not only does the shape of the animal radically change, but so does its physiology: the cerebral ganglion, being primarily for control of movement, serves no purpose to the immobile adult sea squirt, and so the rapidly maturing larva gets to work digesting it for its protein reserves. The tunicate essentially eats its own brain.
Though all adult tunicates are essentially brainless and spineless, they are not heartless or gutless. (Heart, stomach, intestines and gonads are contained in a second, visceral cavity below the water-siphoning atrial cavity.) Nor are all of them passive filter feeders. Deep in the canyons and trenches of the ocean dwell the predatory tunicates (Megalodicopia hians), hooded monsters that wait like Venus flytraps for zooplankton and small animals to drift into their tiny, yawning maws, which then snap shut to capture and digest the hapless creatures.
Most tunicates, however, are benign to the point of hospitable (unless you are plankton). Many sea squirts become homes for fish and shrimp, and animals like the tubesnout fish will lay their eggs in them, which behavior I generally discourage in houseguests.
Likewise, not all tunicates are stay-at-homes. Salps, for instance, are generally colonial tunicates which form long free-floating daisy chains in the ocean. They are the chordate answer to sea jellies: usually transparent, pumping mindlessly in the current as they feed on phytoplankton with their sieves, the salps can travel as individuals, chains, or even swarms, and have a huge effect on the availability of plankton in the oceanic food chain.
Most tunicates, like the salps, are also aggregate or colonial animals cemented together at a common base like tenants of an apartment. But unlike the ethereal salps, most tunicates possess an tough, armored sheath known by the dainty name of the tunic. What makes this tunic sturdier than, say, the tunic of a daydreaming Greek shepherd boy is the presence of two fairly unusual materials to be found in an animal: cellulose and metal. Despite being the most common organic compound on Earth, cellulose is produced by no animal besides the tunicate. That’s because cellulose is the substance gives plant cells their structure. In other words, it’s wood. (To be precise, wood is 50% cellulose. Cotton, however, is 90% cellulose.) In pure crystalline form, tunicates have duplicated the cellulose that strengthens tree trunks to strengthen their own trunks.
But incredibly, the sea squirt’s tunic is also a depository for heavy metals such as lithium, and its blood is not red but rather green, being based on the rare transition metal vanadium. In fact, a tunicate’s body may contain between one hundred and ten million times the amount of vanadium found in the surrounding sea water. How did this ancient creature come to be a vanadium-based life form, and how did it come to share that designation with its non-vertebrate neighbor, the sea cucumber? What the hell is vanadium good for, anyway?
Vanadium (Atomic Number: 23, Atomic Symbol: V) falls between titanium and chromium on the periodic table, and is a soft, silver-grey metal that is never naturally found in its pure elemental state. However, it’s found in over 65 different minerals (primarily vanadinite, bauxite and magnetite), and when combined with iron, steel, or titanium, it significantly strengthens the alloy and can be used to make everything from workman tools to jet engines. (The original Model-T was largely made out of vanadium steel.) In the biological world, vanadium is used almost exclusively by ocean creatures, and few at that. Some algaes require vanadium to produce essential compounds, and some fungi, such as amanita mushrooms, accumulate the metal. But it is otherwise exclusive to the chordate sea squirts and the non-chordate sea cucumbers, bottom dwellers related to sea stars and sea urchins. They use vanabins, proteins which collect the metal to form hemovanadin, the sea squirt and sea cucumber analogue to hemoglobin. In most animals, hemoglobin binds to oxygen and transports it throughout the body. However, there’s no evidence that hemovanadin binds to oxygen at all, and in fact there is already hemocyanin, another oxygen-transporting protein, present in the blood to take care of that business.
It turns out that vanabins don’t even do their job very well. Vanabins so loosely bind to vanadium that captured vanadium is often shed by the cells to travel freely and independently through the bloodstream, turning sea squirts into repository junkyards of rare metal. So why collect vanadium at all? It’s a biological mystery. But my favorite theory is that it uses vanadium as a toxin to deter predators and microbes. In humans, vanadium isn’t highly toxic but does make a handy spermicide. I wonder if filter feeders like sea squirts and sea cucumbers aren’t mining vanadium out of the passing algae in order to neuter the enemy. It may not have a backbone, per se, but the little sea squirt has balls. And now that you’ve eaten a bite of its green-blooded, metallic flesh, you don’t.